It might be important that I used Mesa Rock as opposed to Cobblestone. Also, I don't have access to the world as it is the world of the Unknown Server, which is currently beta testing the dedicated server binary. I probably should have mentioned that as there is a good chance that that might have something to do with the bug.

But it does take some finagling to get an observable amount of vibration.

Edit: It seems that it is so difficult to replicate this bug, that you might not need to bother.